Čaká s ním dieťa. (čaká = she's expecting; s ním = with him; dieťa = baby) / Čaká od neho dieťa. (od neho = from him) - using the preposition od seems to be less common in this case Čakajú spolu dieťa. (spolu = together; lit. "They're expecting a baby together") or just: Čakajú dieťa. Čaká jeho dieťa. (jeho = his; lit. "She's expecting his baby")
Just one correction to the post above: the correct spelling is dijete, not djete.
And there is also another possibility, and it is basically the same one like in Czech: (Ona) čeka s njim dijete.
Although correct, the sentence Ona je trudna s njim can be ambiguous because it can be interpreted in two different ways, depending on the meaning of the pronoun on (< s njim). The pronoun can represent either the baby's father or the baby itself (if it is male).
@Hachi25 ->You are correct, it is dijete and not djete. Thank you for the correction.
As for: " Ona je trudna s njim" can be ambiguous because it can be interpreted in two different ways, depending on the meaning of the pronoun on (< s njim). The pronoun can represent either the baby's father or the baby itself (if it is male).-
---> You are right that the sentence mentioned above can be ambiguous even though in the context of the opening question (She's expecting his baby) it should not lead to confusion.
To avoid any ambiguities: "Ona je trudna s njegovim djetetom"----->She's pregnant with his child.
@ilocas2 -> Your remark is interesting. I am not sure for the other Slavic languages but in Croatian "dijete" (child/baby) is used much more often than "beba" (baby) in all the forms mentioned above when a woman is expecting a child.
The word baby is mostly used by family members and close friends. I wonder if the same goes for other Slavic languages? From reading all the posts in this thread, it looks that way.